Introduction to Genetics Chapter 11

The Work of Gregor Mendel

Genetics The scientific study of heredity.

Vocabulary to Know Trait: a specific characteristic varying among individuals ex: eye color Gene: a portion of DNA determining a trait; found on the chromosomes ex: the gene for eye color

Vocabulary (continued) Hybrid: the offspring of two parents with different traits Gametes: reproductive cells (aka: sex cells; sperm & egg)

Allele: the form of a gene Allele: the different forms of a gene * Represented by a letter. ex: Widow’s Peak = W * Organisms have two alleles for each trait, one inherited from the mother and one from the father. ex: WW

Forms of Alleles Dominant Allele: trait will be expressed with only one copy present * Represented with capital letters. ex: W (Widow’s peak) Recessive Allele: trait will only be expressed when no dominant alleles are present * Represented with lower case letters. ex: w (No widow’s peak)

Gregor Mendel Considered the father of genetics. Mid-1800s Monk who experimented with pea plants in the monastery garden.

Mendel’s Conclusions Inheritance is determined by factors passed from one generation to the next. 2. Principle of dominance: Some alleles are dominant and others are recessive.

Mendel’s Conclusions 3. Segregation: A gamete carries only one copy of each gene. 4. Principle of independent assortment: Genes for different traits segregate independently from one another.

Applying Mendel’s Principles

Allele Combinations Homozygous: both alleles are the same ex: WW or ww Heterozygous: alleles are different ex: Ww (capital letter is always 1st!)

Genotype: the actual allele combination; what the genes say ex: WW, Ww, ww Phenotype: the trait observed ex: Widow’s Peak

Punnett Squares Used to predict the genotypes of offspring when the genotypes of both parents is known.

Other Patterns of Inheritance

Incomplete Dominance The heterozygous phenotype (Ww) is somewhere between both homozygous phenotypes (WW & ww). Ex: pink flowers

Codominance Both alleles contribute to the phenotype; neither allele is dominant . Ex: roan cattle

Multiple Alleles More than two allele possibilities. Ex: blood type

Polygenic Traits Traits controlled by more than one gene. Ex: skin color

Meiosis

Words to Know Haploid: “one set”; cells that contain one set of each chromosome (gametes); 1n Example: in humans n=23, so human haploid cells (gametes) have 1(23) or 23 total chromosomes Diploid: “two sets”; cells that contain two sets of each chromosome ; 2n Example: in humans n=23, so human diploid cells have 2(23) or 46 total chromosomes

Homologous Chromosomes The pair of chromosomes that have the genes for the same traits. A copy is inherited from each parent.

Meiosis A type of cell division that produces gametes containing half the number of chromosomes as a body cell. Contains two separate rounds of division, called Meiosis I & Meiosis II. For example, during meiosis in humans, diploid cells with 46 chromosomes are divided into 4 haploid sex cells (gametes) with 23 chromosomes each. Animation/

Interphase: cell grows, replicates its chromosomes and prepares to divide. Prophase I: chromosomes take shape; homologous chromosomes pair up, forming a tetrad Crossing over can occur! Chromosomes touch & exchange genes. This can mix up linked genes. (ones found on the same chromosome that occur together). Phases of Meiosis

Metaphase I, Anaphase I, Telophase I Nuclear membrane reforms & cytokinesis occurs. Homologous chromosome pairs separate. Chromosomes line up across the cell’s center.

Similar to Mitosis, except 4 haploid daughter cells are created. Meiosis II Similar to Mitosis, except 4 haploid daughter cells are created. Prophase II: No Replication occurs! Metaphase II: Chromosomes line up across the cell’s center.

Meiosis II (continued) Telophase II: Nuclear membrane reforms & cytokinesis occurs. Anaphase II: Chromosomes separate.

The end result: In males, all 4 of the daughter cells become sperm. In females, the division of the cytoplasm is uneven, so only 1 egg is produced. The other three cells, called polar bodies are not used for reproduction.

Genetic Recombination The reassortment of chromosomes and the genetic information that they carry by segregation and crossing over. For example: In humans, n = 23, so the number of different gametes that can be produced is 223 (more than 8 million). When fertilization occurs, 223 x 223, or 70 trillion different zygotes are possible and this is without accounting for crossing over! Zygote: the fertilized egg

Mitosis vs. Meiosis Mitosis Meiosis Two identical daughter cells are formed. Each daughter cell has one complete set of chromosomes. (diploid) Four different gamete cells are created (although in females, only 1 is viable). Each gamete cell has half of the total number of chromosomes. (haploid) http://www.pbs.org/wgbh/nova/miracle/divi_flash.html

Linked genes Genes that are located on the same chromosome & thus, tend to be inherited together.

Human Heredity

Things You May Not Know About DNA Genome: the full set of genetic information carried in an organism’s DNA Humans have 46 chromosomes in their genome. Karyotype: a picture of all diploid pairs of chromosomes, arranged in order of decreasing size

Chromosome Types Autosomes: the remaining 44 human chromosomes other than the sex chromosomes Sex chromosomes: the pair of chromosomes determining gender Males have XY. Females have XX. In female cells, most of the genes in 1 X are randomly switched off (called Barr bodies). Ex: female calico cats

Sex-linked genes A gene located on a sex chromosome. Genes on the Y chromosome are only found in males. Genes on the X chromosome are found in both genders. For recessive traits on the X, they are more common in males than females. Ex: color-blindedness, hemophilia

Pedigree A chart used to show the presence or absence of a trait within a family.

Human Genetic Disorders

Nondisjunction When homologous chromosomes fail to separate during meiosis. Creates monosomy and trisomy.

http://learn.genetics.utah.edu/content/chromosomes/diagnose/ When a Mistake is made…